![background image](http://html2.mh-extra.com/html/national-instruments/gpib-120b/gpib-120b_user-manual_3397767015.webp)
© National Instruments
|
2-3
GPIB-120B User Manual
still in the FIFO buffer. If this situation is unacceptable for your application, you must use
unbuffered mode, in which the 3-wire interlocked behavior of GPIB is maintained.
Buffered mode applies only to data transfers. GPIB command bytes are not stored in the FIFO
buffers; they are transmitted using the GPIB double-interlocked handshaking protocol.
Setting the Data Transfer Mode
To use buffered mode, set the switch to the ON position, as shown in Figure 2-2. To use
unbuffered mode, set the switch to the OFF position.
Figure 2-2.
Switch Setting for Buffered Mode
Note
The switch to select buffered or unbuffered mode is recessed to avoid
unintentional toggling during operation of the GPIB-120B. Flipping the switch may
require a flathead screwdriver or similar tool.
Verify that the switch on your GPIB isolator/expander is in the desired position before powering
on the unit.
Data Direction Control
Bus B sends the data lines to Bus A if the Bus B Source Handshake state is true or if a Controller
on Bus A is conducting a parallel poll.
Bus A sends the data lines to Bus B if the Bus A Source Handshake state is true or if a Controller
on Bus B is conducting a parallel poll.
Parallel Poll Detection
Controllers can conduct parallel polls on Bus A or Bus B, and devices on both Bus A and Bus B
can respond to parallel polls.
If a Controller on Bus A conducts a parallel poll, the parallel poll detection circuitry on side B
conducts a parallel poll on Bus B. The parallel poll result is driven on the data lines of Bus A.
If a Controller on Bus B conducts a parallel poll, the parallel poll detection circuitry on side A
conducts a parallel poll on Bus A. The parallel poll result is driven on the data lines of Bus B.
Refer to the
Parallel Poll Operation
section in this chapter for important information about
conducting parallel polls with the GPIB-120B.
ON
OFF
BUFFERED